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GCN Circular 4403

GRB 051227: Joint Swift-HETE Spectral Analysis
2005-12-28T03:50:13Z (18 years ago)
Don Lamb at U.Chicago <>
GRB 051227: Joint Swift-HETE Spectral Analysis

T. Sakamoto, J-L. Atteia, G. Ricker, N. Kawai, D. Lamb, and S. Woosley,
on behalf of the HETE Science Team;
M. Arimoto, T. Donaghy, E. Fenimore, M. Galassi, C. Graziani,
N. Ishikawa, A. Kobayashi, J. Kotoku, M. Maetou, M. Matsuoka,
Y. Nakagawa, R. Sato, T. Shimokawabe, Y. Shirasaki, S. Sugita,
M. Suzuki, T. Tamagawa, K. Tanaka, and A. Yoshida, on behalf of the
N. Butler, G. Crew, J. Doty, G. Prigozhin, R. Vanderspek,
J. Villasenor, J. G. Jernigan, A. Levine, G. Azzibrouck, J. Braga,
R. Manchanda, G. Pizzichini, and S. Gunasekera, on behalf of the HETE
Operations and HETE Optical-SXC Teams;
M. Boer, J-F Olive, J-P Dezalay, and K. Hurley, on behalf of the HETE

L. Barbier, S. Barthelmy, J. Cummings, N. Gehrels, and J. Norris, on
behalf of the Swift BAT team;


The HETE FREGATE instrument detected the initial peak of GRB 051227
(Swift Trigger 174738; Barbier, et al., GCN 4397) as an untriggered

Carrying out a fit to the FREGATE spectral data alone, the spectrum 
of the initial peak is adequately fit by a power-law (PL) model, with
photon index alpha = 1.3 +0.4/-0.6 (90% confidence level).  This
confirms the result reported by Hullinger et. al (GCN 4400).

Carrying out a joint fit to the BAT plus FREGATE spectral data, the
best-fit cutoff PL model yields an improvement in ch^i2 over the
best-fit PL model of 5.2 for one additional parameter, corresponding 
to a significance of 2.3E-02.  Thus the joint spectral fit provides
evidence of curvature in the spectrum.

Adopting the best-fit cutoff PL model yields a best-fit value of 
Epk = 100 +219/-41.3 keV (90% confidence level) and a fluence ratio 
S(2 keV-30 keV)/S(30 keV-400 keV) = 0.19.  The event is therefore a 
hard GRB.
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